I often see claims that volcanic emissions of sulphur dioxide, on Earth and on the planet Venus, creates sulphuric acid when it combines with water vapour. It doesn't. Sulphur dioxide creates sulphurous acid, which is less corrosive than sulphuric acid. To make sulphuric acid you need sulphur trioxide. Is there a natural process which converts sulphurous acid into sulphuric acid, or has one acid been confused with the other?


Oxidation states of sulphur vary between -2 and +6. Sulphur compounds are generally emitted in reduced form. They are then oxidised in the Earth’s atmosphere, generally to SO2 , a +4 oxidation state. Around 65% (the remainder is removed by dry deposition) of this sulphur dioxide is eventually oxidised to the +6 state of H2SO4 , where SO2 – 4 is the sulphate ion. Sulphur is stable in the presence of oxygen in this state. The higher-state compounds also generally have a greater affinity to water, meaning they are more readily removed from the atmosphere by wet deposition.

The natural atmospheric sulphur cycle and its response to anthropogenic perturbations, Angus J. Ferraro

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  • $\begingroup$ In simple terms, is sulphur dioxide converted to sulphur trioxide by natural processes, and if so, how long does this take? $\endgroup$ – Michael Walsby Aug 5 '19 at 7:50
  • $\begingroup$ Why give complex and long winded answers when a simple answer is all that's required? Example: YES,THERE IS A NATURAL PROCESS WHICH OXIDISES SO2 TO SO3 BY A REACTION WITH OXYGEN IN THE ATMOSPHERE, THIS PROCESS TAKES X NUMBER OF DAYS,WEEKS,MONTHS. $\endgroup$ – Michael Walsby Aug 6 '19 at 11:00
  • $\begingroup$ @MichaelWalsby Ferraro discusses residence time on p.3 . Days in the troposphere, years in the stratosphere. $\endgroup$ – Keith McClary Aug 6 '19 at 14:24
  • $\begingroup$ Thanks. I wonder how it works on Venus, where there is hardly any oxygen in the atmosphere. $\endgroup$ – Michael Walsby Aug 6 '19 at 14:33

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